1. Field of the Invention
The invention relates generally to siloxane-carborane polymers.
2. Description of the Prior Art
Carboranylenesiloxanes are highly sought after high temperature, thermally and thermo-oxidatively stable materials, which have been of particular interest to aerospace and defense industries especially for use in sealing assemblies for landing gears, flight control, and fuel systems and in coating insulations for cables (Dvornic et al., High temperature Siloxane Elastomers, Huthig & Wepf, Heidelberg, Germany 1990). (All referenced publications and patents are incorporated by reference.) There is also a high demand for such materials in the electronic industry in the manufacture of resist layers of computer chips. However, the existing carboranylenesiloxanes are generally low molecular weight polymers, a fact that has precluded their use in applications requiring high molecular weight materials (Papetti et al., “A New Series of Organoboranes 7. Preparation of Poly m-carboranylenesiloxanes,” J. Polym. Sci. Part A-1, 4, 1623 (1966); Mayes et al., “Carborane Polymers 4. Polysiloxanes,” Polym. Sci. Part A-1, 5, 365 (1967)). A solution to this problem was devised by the introduction of unsaturated crosslinkable units in such materials, which on curing yielded extended polymer networks of sufficient strength for various applications (Henderson et al., “Synthesis and Characterization of Poly(carborane-siloxane-acetylene),” Macromolecules, 27(6), 1660 (1997)). In practice, all such derivatives on curing have been known to be converted to plastics, which have thus restricted their use to mainly structural components (Bucca et al., “Oxidation-resistant thermosets derived from thermal copolymerization of acetylenic monomers containing boron and silicon,” J. Polym. Sci. Part A: Polym. Chem., 37(23), 4356 (1999); Homrighausen et al., “Synthesis and characterization of a silarylene-siloxane-diacetylene polymer and its conversion to a thermosetting plastic,” Polymer, 43(9), 2619 (2002)). However, in applications involving high temperature coatings, sealings, composites, etc., there is a need for elastomeric materials. There is also a need for the curing to be effected in an expeditious manner under an ambient or inert atmosphere. The existing methodologies for the curing of carboranylenesiloxanes with unsaturated internal or terminal crosslinkable groups are by thermal crosslinking of the unsaturated groups or by the crosslinking of these groups by the use of chloroplatinic acid (H2PtCl6·6H2O), a hydrosilation catalyst (Houser et al., “Hydrosilation routes to materials with high thermal and oxidative stabilities,” J. Polym. Sci. Part A: Polym. Chem., 36(11), 1969 (1998); U.S. Pat. Nos. 5,981,678 and 6,225,247). The thermal curing reaction requires a temperature between 250-400° C. for several hours, and the hydrosilation reaction using chloroplatinic acid requires several hours or days. These constraints inherent in the existing systems, in addition to the alluded tendency of the materials to be converted into plastics on curing, consequently have precluded the use of these systems in elastomeric applications.
It is believed that the inability to control the extent of curing in these materials is the main reason for their plasticity on curing, as it results in inordinately extensive crosslinked systems. For example, on thermal curing a known polymer poly(carboranesiloxane-acetylene) is so extensively crosslinked that it inevitably converts into a plastic. Houser and Keller reported previously that an extensively crosslinked system was produced from reaction of divinyl-terminated carboranesiloxane containing compound with poly(methylhydrosiloxane) in the presence of a hydrosilation catalyst resulting in the formation of a plastic. In this case, the unsuitability of the product's characteristic (brittleness) is compounded by the fact that a reaction time of several days was required to complete the curing. This is due to an inherent deficiency in the catalyst, which is an outcome of its associated mechanism. It has been well established that a hydrosilation catalyst such as chloroplatinic acid functions as a heterogeneous catalyst (Lewis et al., “Platinum-catalyzed hydrosilylation-colloid formation as the essential step,” J. Am. Chem. Soc., 108(23), 7228 (1986)). The Pt metal is converted into a colloidal form during the induction step and the catalysis occurs at the colloidal Pt. Unfortunately, chloroplatinic acid forms larger colloidal particles compared to other heterogeneous hydrosilation catalysts such as the Karstedt catalyst, Pt[COD]2, etc, which can form very fine Pt colloids. Thus, the latter catalysts are infinitely superior to chloroplatinic acid, and hence, facilitate hydrosilation reactions in an expeditious manner. It is also known that these heterogeneous hydrosilation catalysts require the presence of O2 to perform hydrosilation and hence, would not be effective as catalysts under an inert atmosphere (Lewis, “On the mechanism of metal colloid catalyzed hydrosilylation: proposed explanations for electronic effects and oxygen cocatalysis,” J. Am. Chem. Soc., 112(16), 5998 (1990)). For applications that have to be carried out under an inert atmosphere such as in composite fabrication or repair, this necessity of the heterogeneous catalysts for O2 precludes their use as a catalyst in such applications.
In the literature, there is a plethora of examples of homogeneous hydrosilation catalysts especially of Pt and Rh metals (Skoda-Foldes et al. “Homogeneous Catalytic Hydrosilylation of the C═C Double Bond in the Presence of Transition-Metal Catalysts,” J. Organomet. Chem., 408(3), 297 (1991)). These catalysts have been established to perform hydrosilation reactions under an inert atmosphere. For example, homogeneous catalysts such as Pt(PPh3)4, PtCl2(PPh3)2, RhCl(PPh3)3, RhCl3·3H2O, Rh(PPh)3Cl, etc. are known to facilitate a wide range of hydrosilation reactions under an inert atmosphere. Some of these reactions, even though being not as fast as the ones by heterogeneous catalysts such as Karstedt or Pt(COD)2, do proceed at an appreciable rate. Another homogeneous catalyst [Rh(COD)Cl]2 is known to catalyze the hydrosilation of butadiynes, which are close analogues of diacetylenes (Kusumoto et al., “Hydrosilylation of 1,4-Bis(trimethylsilyl)-1,3-butadiyne,” Chem. Lett. 9, 1405 (1985); Tillack et al., “Hydrosilylierung von symmetrisch disubstituierten Alkinen und Butadiinen mit L2Ni(0)-Butadiin-Komplexen [L=Ph3P, ((o-Tol-O))3P] als Katalysatoren,” J. Organomet. Chem., 532(1-2), 117 (1997); Tillack et al., “Catalytic Asymmetric Hydrosilylation of Butadiynes: A New Synthesis of Optically Active Allenes,” Tetrahedron Lett., 40(36), 6567 (1999)).
An example of a homogeneous hydrosilation catalyst that affects the catalysis at a rate that is comparable to that of heterogeneous hydrosilation catalysts is Pt(acac)2, which functions under photochemical conditions. It is known to expediently and efficiently catalyze the hydrosilation of olefins in the presence of wavelengths of >350 nm (Lewis et al., “Platinum(II) Bis(β-diketonates) as Photoactivated Hydrosilation Catalysts,” Inorg. Chem., 34(12), 3182 (1995); Wang et al., “Photoactivated hydrosilylation reaction of alkynes,” J. Organomet. Chem., 665(1-2), 1 (2003)). An irradiation of an olefin and silane mixture in CH2Cl2 containing the catalyst with wavelengths of >350 nm for 10 min at ambient temperature is found sufficient to cause a high conversion of the olefin to the hydrosilated product. Another photochemical catalyst, which causes the hydrosilation of olefins expediently at mild temperatures, is Fe(CO)5 (Randolph et al. “Photochemical reactions of (η5-pentamethylcyclopentadienyl)dicarbonyliron alkyl and silyl complexes: reversible ethylene insertion into an iron-silicon bond and implications for the mechanism of transition-metal-catalyzed hydrosilation of alkenes,” J. Am. Chem. Soc., 108(12), 3366 (1986)).